python - 如何将函数与 pandas 数据框一起使用

Question

如何将函数与 pandas 数据框一起使用。例如:

a       | b
london  | uk
newyork | usa
berlin  | germany

df1 = df[['a', 'b']]

def doSomething(df1):
    return df1

doSomething() 将返回两列 a 和 b，但是如何仅返回 a ？

def doSomething(df1):
    return df1.a 

df1.applymap(doSomething)

AttributeError: ("'str' object has no attribute 'a'", u'occurred at index a')

Answer 1

您可以使用:

df = pd.DataFrame({'a':['london','newyork','berlin'],
                   'b':['uk','usa','germany'],
                   'c':[7,8,9]})

print (df)
df1 = df[['a', 'b']]

def doSomething(x):
    return x.a

#function works with DataFrame 
print (doSomething(df1))
0     london
1    newyork
2     berlin
Name: a, dtype: object

#function works with Series, columns are transformed to index of Series
#return for each row value of Series with index a which is transformed to column in output df
print (df1.apply(doSomething, axis=1))
0     london
1    newyork
2     berlin
dtype: object

如果需要applymap它适用于 df 的每个元素:

def doSomething(x):
    return x + '___'

#function works with element
print (df1.applymap(doSomething))
            a           b
0   london___       uk___
1  newyork___      usa___
2   berlin___  germany___